1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device and a method of driving the semiconductor memory device.
2. Related Art
In recent years, microfabrication of nonvolatile memories such as NAND flash memories has increasingly proceeded. When an interval between adjacent memory cells is narrower to follow microfabrication in machining, proximity effect between the memory cells becomes conspicuous. The proximity effect is interference affected by the memory cell from an adjacent memory cell by capacitive coupling, and the like, between a plurality of adjacent memory cells. For example, charges accumulated in a memory cell adjacent to the memory cell influence data stored in the memory cell. As the interval between the adjacent memory cells is narrower, the influence affected by the charges of the adjacent memory cell becomes greater.
To correctly read the data stored in a memory cell, it is necessary that a threshold voltage of the memory cell falls within a predetermined range. Normally, right after data is written, a threshold voltage of a certain memory cell remains within a predetermined range. However, when the data is thereafter written to the adjacent memory cell, the threshold voltage of the memory cell fluctuates by the proximity effect. This causes erroneous reading of data.
For example, in a multivalued memory cell capable of storing data of two bits or more, a permissible range for the threshold voltage of the memory cell is very narrow. The capacitive coupling between the adjacent cells by the proximity effect expands a threshold distribution, and thus there is a need of much narrowing the threshold distribution of the memory cell than that of a binary memory. Accordingly, in a conventional program operation (write operation), a step-up voltage needs to be small, and the number of steps to reach a desired write threshold value increases. As a result, there is a problem that a programming time is prolonged (write rate becomes slow).